An air-core coil, formed of conductive wires with insulation coating, embedded in a magnetic powder is nominated as a conventional choke coil that has been used until today. (Japanese Patent Unexamined Publication No. 2002-246242 discloses an example in FIG. 12 on page 1). The choke coil has a structure such that metal terminals are coupled at ends of the air-core coil by welding, soldering or bonding with a conductive adhesive.
Along with the recent trends of downsizing and low profiling in electronic equipment, a more downsized and low profile designing is required, and in response to a higher speed and integration in LSI such as CPU or the like, a larger current capacity of several A to several tens A in high frequency range is also required for inductors such as choke coil or the like.
Therefore, an excellent and cheaper inductor is awaited that has lower resistance to suppress the heat generation owing to the downsizing, lower losses in high frequency range and lesser decrease of inductance in larger current range caused by DC superposing.
Along with the trends of downsizing and low profiling in electronic equipment, a variety of power supply circuits have been developed in the field of DC/DC converter.
A circuit system for instance so-called multi-phase system to drive a plurality of DC/DC converters in parallel by phase control, as shown in FIG. 4, can reduce ripple currents and can provide a high frequency large current with high efficiency.
A transformer system, as shown in FIG. 6, connecting an intermediate tap provided in the choke coil to a switching element is said to contribute greatly to the design freedom in electronic devices or voltage conversion efficiency in addition to the above needs.
However, the above mentioned circuit configuration alone is not necessarily enough to realize a large current system in high frequency range but choke coils used in the power supply circuit should preferably be designed to realize downsizing and available for high frequency large current applications.
The structure of above-mentioned conventional choke coil, however, metal terminals and intermediate taps must be coupled afterward, and therefore, can hardly keep the DC resistance in a low level. Additionally, the structure not only needs a large setting space but also considered disadvantageous in production cost in case of employing the multi-phase system, the transformer system being introduced for future or a combination system of the two systems.